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Ethane is an alkane and as such is saturated, meaning it has all single bonds. Ethene is an alkene and as such is unsaturated, meaning it has one carbon to carbon double bond. So they can be identified by their displayed formulae, as ethene has the double bond functional group.

Chemically, they can be distinguished most easily by the reaction with bromine water, which is orange/red in colour. Add a few drops to ethene and bromine water is decolourised as 1,2-dibromoethane, a saturated compound forms. There is no visible sign of reaction when bromine is added to an alkane.